I've got a "jiggle the ball" issue with my Duotron that happens rarely enough that I have not spent a lot of time trying to resolve yet - usually a little nudging (not enough to tilt) triggers an eject. I've been at that ball sensor switch a few times - it's been too sensitive at times (triggers randomly due to game vibration) and not sensitive enough (does not reject reliably)

I'd also say if it happens to both holes it's less likely to be the sensor switch and to look elsewhere.